1. Field of the Invention
This invention relates to a cascade cryogenic thermoelectric cooler and a method of manufacturing the same. The cascade cryogenic thermoelectric cooler integrates high coefficient of performance thin-film super-lattice devices with cascaded bulk-material-based thermoelectric devices to enable cooling to cryogenic temperatures such as 30-120 K.
2. Discussion of the Background
Solid-state thermoelectric cooling to cryogenic temperatures between 70 and 120 K will improve the performance of electronics and sensors such as for example RF receiver front-ends, infrared (IR) imagers, ultra-sensitive magnetic signature sensors, and superconducting electronics based on high-Tc (100 to 120 K) superconducting materials.
Today, bulk thermoelectric materials based on p-BixSb2xe2x88x92xTe3 and n-Bi2Te3xe2x88x92xSex do not have a sufficient figure-of-merit (ZT) or a coefficient of performance (COP) to achieve cryogenic temperatures. For example, a commercial 6-stage Melcor thermoelectric cooler (Melcor, Trenton, N.J.) with a COP of about 0.028 can only approach a cold-side temperature of about 167 K for a hot-side temperature of 300 K. Similarly a 6-stage Marlow thermoelectric cooler (Marlow Industries, Dallas, Tex.) can approach a temperature of about 165 K with a COP of 0.026.
The principle reason that thermoelectric devices with a hot-side of 300 K based on bulk p-Bi2xe2x88x92xSbxTe3 and bulk n-Bi2Te3xe2x88x92ySey can not approach cryogenic temperatures is that the ZT values of bulk materials drop as the temperature lowers. The figure of merit drops at lower temperatures because of a higher thermal conductivity as well as a lower Seebeck coefficient.
One bulk-material which does not have low ZT values at lower temperature is BiSb A BiSb device could be stacked on top of a cooler made from bulk p-Bi2xe2x88x92xSbxTe3 and bulk n-Bi2Te3xe2x88x92ySey. However, for BiSb to offers a reasonable ZT, in order to achieve cryogenic temperatures, a magnetic field must also be used; this is not practical in most applications. Furthermore, both n- and p-type conducting BiSb materials are not achievable.
In essence, there are no set of known bulk thermoelectric materials (certainly not devices) that have sufficient ZT (and COP in devices) between 85 and 300 K to achieve cryogenic refrigeration.
In contrast to bulk materials, the thermal conductivity of superlattice structures decreases at lower temperatures. A variety of processes in superlattice structures such as for example mini-band conduction, lack of alloy scattering, and interface-carrier-scattering apparently better preserve reasonable Seebeck coefficients at lower temperatures. Thus, superlattice materials are expected to have at lower temperatures higher ZT values than bulk-materials, and devices made from superlattice materials are expected to have higher COP. Despite the higher ZT of superlattice thin-film materials, thin film cryogenic thermoelectric coolers are not available. Integration of a large number of superlattice thin-film device stages necessary to achieve the temperature difference between room and cryogenic temperatures presents complications which are beyond the maturity of superlattice thermoelectric devices, presently limited by thermal mismatch and temperature gradient issues and also practically limited by the high cost of thin-film superlattice materials.
Thus, an all-thermoelectric cryogenic cooler, implying the advantages of solid-state reliability and without additional mechanical or other forms of cooling, is not available.
Accordingly, one object of the present invention is to provide a cascade cryogenic thermoelectric cooler integrating a bulk-material based thermoelectric cooler with a super-latticed thermoelectric cooler. The bulk-material based thermoelectric cooler is configured with a cascade of multiple stages with each stage configured to cool to progressively lower temperatures, and the super-latticed thermoelectric cooler is interfaced to the bulk material device thermoelectric cooler.
Another object of the present invention is to provide a cascade cryogenic thermoelectric cooler which can approach a cold side temperature of 85 K.
Still another object of the present invention is to interface a super-lattice thin film thermoelectric cooler with a bulk-material-based thermoelectric cooler such that the bulk-material-based thermoelectric cooler reduces the hot-side temperature of the super-lattice thin film thermoelectric cooler to significantly below 300 K, for example between 170-200 K, wherein super-lattice materials relying on the thermal conductivity reduction due to phonon scattering at the super-lattice interfaces will be more efficient.
A further object of the present invention is to reduce the thermal mismatch and temperature gradients imposed on a cascade of super-lattice thin-film coolers.
Another object of the present invention is to provide a thermoelectric cooler wherein the potentially expensive super-lattice technology is utilized only for achieving cryogenic or near-cryogenic temperatures and thus provides a cost-effective cryogenic cooler.
Still another object of the present invention is to provide an integrated thermoelectric cooler in which high performance/high ZT superlattice structure thin-film thermoelectric devices could be used to more efficiently cool than a thermoelectric cooler using only bulk-materials.
These and other objects are achieved according to the present invention by providing a novel cascade thermoelectric cooler designed to cool to cryogenic temperatures of 30 to 120 K. The cascade thermoelectric cooler integrates high performance high-ZT BixSb2xe2x88x92xTe3 and Bi2Te3xe2x88x92xSex-based super-lattice-structure thin-film thermoelectric devices with a bulk-material based thermoelectric cooler including plural cascaded cold stages with each successive cascaded cold stage able to cool to a progressively lower temperature. Each cold stage in the bulk-material thermoelectric cooler includes a heat source plate, a heat sink plate, p-type thermoelectric elements, and n-type thermoelectric elements. Moreover, the thin film thermoelectric cooler can have multiple stages which each stage contains a heat source plate, a heat sink plate, p-type super-latticed thermoelectric elements, and n type super-latticed thermoelectric elements. By attaching an output heat source plate on the thin-film thermoelectric cooler to an input heat sink plate on the bulk-material thermoelectric cooler, the integration of the thin film thermoelectric with the bulk-material-based thermoelectric yields a cascade thermoelectric cooler wherein the bulk-material-based thermoelectric cooler cools to 170-200 K and the thin-film thermoelectric device cools to cryogenic temperatures between 70 and 120 K. Another level of thin-film super-lattice integration can achieve temperatures near 30 K.
According to one aspect of the present invention, the cascade thermoelectric cooler is utilized to cool superconducting coils in electric motors or generators. The cascade cooler is either integrated directly in contact with the superconducting coils or mounted to a sub-77 K transfer coupling in thermal contact with the superconducting coils. The cascade cooler either cools through multiple stages from near room temperature to cryogenic temperatures or cools from liquid nitrogen temperatures (i.e. 77 K) to cryogenic temperatures.